Metal cans are generally coated on interior and exterior surfaces thereof to be protected from corrosion. Recently, the development of methods for obtaining corrosion resistance without using an organic solvent has been promoted for the purpose to simplify production process, improve sanitation and prevent pollution. As one of such methods, coating of a metal can with a thermoplastic resin film has been attempted.
That is, studies on a method for making cans by laminating a thermoplastic resin film onto a metal plate such as tinplate, tin-free steel or aluminum and then drawing the laminated metal plate are under way.
It is becoming clear that a copolyester film is suitable for use as this thermoplastic resin film in view of moldability, heat resistance, impact resistance and flavor retention properties. However, the polyester film does not always exhibit sufficient taste and flavor retention properties when a can coated therewith contains a drink of which very delicate taste is important, such as green tea, or mineral water which must be tasteless and unflavored, and changes in flavor and taste are detected.
Japanese Laid-open (Kokai) patent application No. 6-116376 proposes a polyester film to be laminated onto a metal plate and molded, which comprises a copolyester containing alkali metal elements and germanium element in specific amounts and has improved flavor retention properties. This film exhibits excellent taste and flavor retention properties when the container with the content is not heated, like the case of a cold pack system, but the film does not always obtain sufficient taste and flavor retention properties when the container with the content undergoes heat treatment, like the case of a retort treatment.
Japanese Laid-open (Kokai) patent application No. 8-231690 proposes a copolyester film comprising terephthalic acid as a main acid component and 1,4-cyclohexanedimethanol and ethylene glycol in a specific proportion range as diol components. Although this film has been proposed to obtain flavor retention properties for contents which require a retort treatment, it cannot obtain sufficient can making properties due to its low heat resistance.
It is an object of the present invention to provide a polyester film to be laminated onto ametal plate and molded, which has overcome the defects of conventional technology and has improved flavor retention properties for contents, particularly taste and flavor retention properties after a retort treatment, while retaining excellent moldability, heat resistance and impact resistance of a copolyester film.
Other objects and advantages of the present invention will become clear from the following description.